FIGS. 10, 11 and 12 show the construction of prior art zero bias type convolvers having a piezoelectric film/insulator/semiconductor structure.
FIG. 10 is a perspective view of a zero bias type convolver, FIG. 11 is a cross-sectional view thereof, and FIG. 12 is a cross-sectional view of a zero bias type convolver having another construction. In these figures, reference numeral 1 is a piezoelectric film; 2 is an insulator; 3 is a semiconductor; 4 is a p or n conductivity type semiconductor layer; 5 is an n or p conductivity type semiconductor layer; 6 is a semiconductor epitaxial layer; 7 is a gate electrode; 8 is a rear electrode; 9 is interdigital electrodes of an input transducer; 10 is a high impurity concentration n or p conductivity type semiconductor substrate; 11 is an input terminal; and 12 is an output terminal.
FIGS. 11 and 12 represent different constructions for the semiconductor 3. In the construction indicated in FIG. 11, the semiconductor 3 has a three-layered structure of p conductivity type semiconductor/n conductivity type semiconductor/high impurity concentration n conductivity type semiconductor substrate or n conductivity type semiconductor/p conductivity type semiconductor/high impurity concentration p conductivity type semiconductor substrate. The two semiconductor layers (4 and 5) formed on the high impurity concentration n or p conductivity type semiconductor substrate 10 are semiconductor epitaxial layers 6 formed on the semiconductor substrate 10 by depletion and in many cases the semiconductor uppermost layer 4 is formed by the ion implantation method. The construction indicated in FIG. 11 represents a typical zero bias type convolver. This is because it is possible to set the gate voltage (working voltage), for which the convolution efficiency Ft of the convolver is highest, in the neighborhood of zero volt, by the fact that only the semiconductor uppermost layer 4 has a conductivity type, which is opposite to that of the other semiconductor layers (5 and 10). Concerning the detail on the construction indicated in FIG. 11, refer to following literatures [1] and [2].
Literature [1]
Syuichi MITSUTSUKA, etc. "Trial fabrication of a zero bias drive type monolithic ZnO/SiO.sub.2 /Si convolver" Preliminary Report of Autumn Meeting 1986 of Applied Physical Society of Japan, P. 905
Literature [2]
JP-A-Sho 62-64113 (laid open Mar. 23, 1987)
On the other hand, in the construction indicated in FIG. 12, the semiconductor 3 has a two-layered structure of semiconductor epitaxial layer/high impurity concentration semiconductor substrate. The high impurity concentration semiconductor substrate is the n or p conductivity type semiconductor substrate 10. The gate voltage (working point) when the convolution efficiency Ft is highest is at a value other than zero volt in an ideal state, where there exists no fixed electric charge in the insulator 2 and further the interfacial level density at the interface of insulator/semiconductor is negligibly low. Therefore, in an ideal element having the construction indicated in FIG. 12, when the gate voltage (voltage between the gate electrode 7 and the rear electrode 8) is at zero volt, the convolution efficiency Ft is low. However, in a real element, the gate voltage when the convolution efficiency Ft is highest can be in the neighborhood of zero volt, because fixed electric charge enters the insulator 2 or interfacial levels are formed in the process for forming the piezoelectric film 1 (for which the sputtering method, the CVD method, etc. are used). In such a case, zero bias drive is made possible even with the construction indicated in FIG. 12. Concerning the detail on the construction indicated in FIG. 12, refer to following literatures [3] and [4].
Literature [3]
JP-A-Sho 63-62281 (laid open Mar. 18, 1988)
Literature [4]
JP-A-Sho 63-197111 (laid open Aug. 16, 1988)
The SAW convolver indicated in FIG. 10, as explained above, is sealed in a package at practical use, similarly to a usual SAW filter, taking resistance to environment and handling into account.
FIG. 13 shows an example of the prior art package construction for the SAW convolver. In the figure, 13 is an SAW convolver; 14 is a shielding electrode; 15 is a sound absorber; 16 is a cover of the package; 17 is a base plate of the package; 18 is an input signal pin; 19 is an output signal pin; and 20 is a ground pin.
In the SAW convolver 13 in FIG. 13, shielding electrodes 14 and sound absorbers 15 omitted in FIG. 10 are indicated. Each of the shielding electrodes 14 is located between each set of interdigital electrodes 9 and the gate electrode 7 and grounded within the package, as indicated in FIG. 13 (connected with the base plate of the package through a bonding wire). The shielding electrodes 14 are disposed for preventing that a part of the input signal inputted to the interdigital electrodes 9 leaks directly to the gate electrode 7 through electromagnetic coupling so that a part of the input signal is superposed on the convolution output signal. Since these shielding electrodes are well known for the SAW convolver element, they are no specifically shown in the construction indicated in FIG. 10. Further the sound absorbers 15 are disposed for preventing unnecessary reflected wave of the surface acoustic wave from the end surfaces of the SAW element. Since these are also well known for the SAW element, these are not shown in the construction indicated in FIG. 10.
In the prior art are package indicated in FIG. 13, the base plate 17 of the package is made of metal and the SAW convolver 13 is mounted on the base plate 17 described above so that the rear electrode 8 of the convolver and the base plate 17 are connected electrically and in addition secured mechanically to each other (die bonding process). Usually the die bonding process is effected often by using conductive adhesive. Consequently the base plate 17 of the package serves as a ground plane for the convolver. At this time, the input signal pins 18 and the output signal pins 19 are insulated electrically from the base plate 17 and the output signal pins are connected with a plurality of points on the gate electrode 7 of the convolver. Further, on the package there is disposed a ground pin 20 connected electrically with the base plate 17 apart from the input signal pins described above. The cover 16 of the package is usually made of metal similarly to the base plate. The cover 16 and the base plate are welded usually by the electric resistance welding method, filling the package with inert gas such as N.sub.2 gas, so that the package is hermetically sealed.
If the package construction indicated in FIG. 13 is utilized for the package of the zero bias type SAW convolver as indicated in FIG. 10, following problems are produced.
In the SAW convolver having a piezoelectric film/insulator/semiconductor structure, when a bias voltage is applied directly between the gate electrode 7 and the rear electrode 8, injection or emission of electric charge in or from the piezoelectric film 1 is produced. When such injection or emission of electric charge is produced, the working point (gate voltage, for which the convolution efficiency Ft is highest) of the convolver is generally shifted. Consequently, in the zero bias type SAW convolver, even in the case where the working point is originally in the neighborhood of zero volt, when a DC bias voltage is applied thereto as described previously, injection or emission of electric charge in or from the piezoelectric film is produced so that the working point is shifted to a voltage other than those in the neighborhood of zero volt. In such a case, even if the gate voltage is set at zero volt in the zero bias type SAW convolver, the convolution efficiency Ft decreases remarkably with respect to the original value thereof. Such decrease of the convolution efficiency Ft continues, until the injected or emitted electric charge is again emitted or injected so that the thermal equilibrium state before the application of the DC bias voltage is reestablished. However, at a temperature below the room temperature, since the resistivity of the piezoelectric film 1 is generally great, a period of time longer than at least several hours is required often, before electric charge, which has been once injected or emitted, returns so that the electric charge distribution in the original thermal equilibrium state is reestablished.
When the characteristics of the zero bias type SAW convolver indicated in FIG. 10 as described above is considered, in the prior art package construction indicated in FIG. 13, since the output pins 19 are insulated electrically from the base plate 17 of the package acting as the ground plane, there is a risk that a voltage due to electrostatic charge or an accidental voltage due to erroneous handling of the package is applied thereto. Since such a voltage is applied thereto, as described previously, even for the SAW convolver, with which a high convolution efficiency Ft can be obtained originally at zero volt, only a low convolution efficiency Ft can be obtained in a long period of time. That is, in the case where the prior art package construction indicated in FIG. 13 is used for the zero bias type convolver, it has a problem in the stability for a long period of time or the reliability of the characteristics of the SAW convolver.